atiext: Add size to bios table for radeon Linux driver (KMS)

The Linux radeon KMS driver does an additional check when looking up COMBIOS
table entries. It checks a header size field to validate that the entry
falls in the header region. Prior to this patch that size field is zero
which causes the driver to fall back to another method (CLOCK_CNTL
registers) also resulting in bad values and a division by zero.

Tested on Debian Squeeze (Linux 2.6.32) and Debian Trixie (Linux 6.12.94)
with:

qemu-system-x86_64 -vga none -device ati-vga,model=rv100,vgamem_mb=32

Signed-off-by: Chad Jablonski <chad@jablonski.xyz>